Tetrode keying system



Oct. 16, 1951 T. A. BIRCKHEAD TETRODE KEYING SYSTEM Filed Sept. 9, 1949 5000V.D.C.

INVENTOR Taylor A.Birckhead.

WITNESSES:

Patented Oct. 16, 1951 UNITED STATES PATENT OFFICE TETRODE KEYIN G SYSTEM Taylor A. Birckhead, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 9, 1949, Serial No. 114,755

8 Claims. i

This invention pertains to electronic apparatus, and particularly to improvements in the control of electronic oscillators.

A principal object of the invention is to provide an apparatus for controlling the operation of a radio frequency oscillator, such as may be used in industrial heating apparatus, and by which the initiation and termination of the radio frequency power output may be simply, accurately and economically controlled.

. A further object of the invention is to provide a single unit which is arranged to supply the proper bias voltages required for the operation of a tetrode oscillator, and which at the same time will serve as an eflicient keying unit for initiating and terminating the oscillating condition of such apparatus.

Still another object of the invention is to provide a single bias supply and keying unit which will simultaneously furnish the proper voltages for control of both the screen grid and the control grid of a multi-grid oscillator.

Another object of the invention is to provide a single self-contained unit which is capable of supplying the above-mentioned direct current bias and control voltages from a generally available source of alternating current energy, such as an industrial power supply line or the like.

A further object of the invention is to provide a bias and control unit for high power radio frequency oscillators that will enable relatively large amounts of radio frequency energy to be switched on and off through the use of a minimum amount of controlling energy during both "on and on phases of operation of the oscillator producing such radio frequency energy.

The invention itself, and a preferred manner of accomplishing the desired results, will best be understood from the following detailed specification of a preferred embodiment thereof, taken in connection with the appended drawing. For purposes of illustration, the invention is there shown as applied to the control of a single-tube tetrode oscillator of a type useful in electronic industrial heating apparatus, but it is to be understood that the principles of the invention may equally well be applied to multi-grid oscillators for other applications, and also to multi-tube tetrode oscillators of forms well known in this art.

Referring now to the drawing, whose single figure illustrates the application of the invention to an electronic oscillator of the type just referred to, reference numeral l0 designates a tetrode oscillator tube provided with a thermionic cathode [2, a control grid [4, a screen grid l6, and a plate or anode I 8, these elements being contained within an evacuated envelope for operation in a usual and well known manner as a screen grid oscillator. The plate I8 is indicated as supplied with a constant direct current potential (which may for example be of the order of 5,000 volts or more), and the cathode i2 is shown as grounded to complete the space-discharge circuit through the source of plate supply voltage. It will be understood that usual and well known means are provided for heating the cathode E2 to produce the thermionic emission, in a manner well known in the art.

Conventional triode oscillators have found wide application as generators of both high and low frequency electric energy, and the prior art is replete with circuit arrangements adapted for the control of the output of such triode oscillators by controlling the potentials applied to the various electrodes thereof. For many applications, however. the power and frequency requirements necessitate the use of thermionic oscillators having an additional grid, generally denoted a screen grid, between the control grid and plate of the tube and supplied with a positive direct current potential usually somewhat below that applied to the plate electrode. This screen grid operates to reduce the effective plate-to-grid capacitance of the tube, thereby extending the frequency at which such an oscillator may be used to deliver its maximum power output. Such a tetrode oscillator has additional advantages which need not, for the purpose of disclosing the present invention, be elaborated herein.

In utilizing tetrode vacuum tubes as amplifiers, the screen grid is normally provided with a fixed positive direct current bias and has no radio frequency voltage applied thereto, while the operation of such a tetrode as an oscillator requires that the screen grid and control grid potentials vary concomitantly in accordance with the radio frequency voltage. Variation or swing in screen grid potential must be of a very considerable amplitude, while the control grid voltage usually has somewhat less swing than in a triode oscillator. The reason for the large swing in screen grid voltage of a tetrode oscillator arises from the necessity for such bias voltage to swing from the high positive value necessary to start oscillation, to a negative value during operation. In addition, it is found that the amount of negative bias required in the control grid to key the oscillator tube to off condition can be materially reduced if the positive bias on the screen grid is simultaneously lessened.

In view of the above, a. combined bias voltage supply and keying unit must be arranged to have the following capabilities: (l) to supply negative control grid bias; (2) supply a positive screen grid bias which can swing negative during operation, (3) drive the control grid more negative to cut off the tube without excessive positive potential on the screen grid, and (4) return the normal direct current bias on the screen grid to its desired positive value when cut-01f Voltage is removed from the control grid to produce normal oscillation.

A bias supply and keying unit fulfilling the above requirements in a novel and efiicient manner is illustrated in the drawing by the parts enclosed within the dotted rectangle 20. As there shown, a supply transformer 22 whose primary winding is energized from a normal power line source, has its secondary winding connected to the respective anodes of a pair of rectifiers 24, 26 (which may be diode vacuum tube rectifiers, or of any equivalent type) arranged as a full wave rectifier. The common cathode connection of the rectifiers thus forms the positive conductor of the bias voltage supply, the negative conductor being connected to the center tap of the transformers secondary winding. A usual and well known form of ripple filter may comprise inductor 23, 30 and condensers 32, 35, the direct current voltage thus obtained being applied across a voltage divider comprising resistors 36 and 33, whose common connection is grounded as at 40 so as to remain at the same potential as the cathode 12 of oscillator Hi. The positive end 02 of the voltage divider is connected through a suitable dropping resistor 44' to the screen grid [6 of tetrode l0, and the negative end of divider resistor 38 is connected through another dropping resistor 40 to the control grid i l of said tetrode. The screen and control grids of the oscillator are both bypassed to ground potential by suitable capacitors i8 and 50 in a well known manner.

Numeral 52 designates the contacts of a keying relay having an operating coil 54, the contacts being connected between the common connection of divider resistors 36 and 38 and the negative potential end of the latter. It is clear that with contacts 52 closed, resistor 38 is shortcircuited and the full voltage of the rectifierfilter combination is across resistor 36. In the example chosen by way of illustration, this voltage may be of the order of a thousand volts, the value of the voltage and of resistance 44 being chosen so as to provide a normal screen grid current of 100 ma. D. 0., producing a drop or 1,100 volts, resulting in a negative bias voltage on the screen grid (with respect to the cathode 12) of the order of 100 volts. A control grid current of 50 ma. through the grid resistor 40 (which would have a value of 4,000 ohms in the embodiment disclosed) would produce a negative control grid bias of the order of 200 volts. Under these 0011-. ditions, the tube ID will oscillate normally. If desired a potentiometer may be substituted for the resistors 36 and 38.

To key the oscillator off contacts 52 are opened. Immediately, the potential of the positive end of resistor 36 with respect to ground drops from a thousand volts to ,600 volts, since the opening of contacts 52 removes the shortcircuit around resistor 38. At the same time the control grid is driven more negative by the additional 400 volts appearing across resistor38, so

that oscillation of tube ceases and additional biases from the grid resistors 44 and 46 are removed, leaving the oscillator tube [0 dormant with its screen grid at a potential of plus 600 volts and its control grid at a potential of minus 400 volts.

In order to key the oscillator to on condition all that is necessary is to close contacts 52. This performs two functions simultaneously: (1) negative bias is removed from the control grid to permit oscillation, and (2) the screen grid potential is jumped from plus 600 volts to plus 1,000 volts to insure the starting or oscillations. At the same time, the respective potential-drops across grid resistors 44 and 40, resulting from flow of grid currents, allow the grid of tube 10 to assume their normal operating potentials.

For the sake of illustration, the following values are tabulated as exemplary of suitable arrangements for the control of a tetrode l0 supplied with 5,000 volts direct current on its plate:

D. C. output voltage of bias supply rectifier volts' 1,000 Resistor 36 ohms 12,000 Resistor 38 do 8,000 Resistor 44 do 11,000 Resistor 46 do 4,000

'and closing of a single pair of contacts, with numerous advantages from both constructional and operational viewpoints. It is to be understood, however, that the details of the circuit arrangement described above are intended by way of example and not of limitation, since they may be varied by those skilled in this art to suit particular applications, without departing from the spirit of the invention as defined in the appended claims.

I claim as my invention:

1. A bias voltage supply and keying control unit for use with multi-grid oscillator tubes, comprising a source of direct current potential at least equal to the maximum bias voltage required for any of saidgrids, a voltage divider network connected across said source, connections from said source adapted for connection to a pair of such grids, and circuit closing means for selectively short-circuiting a portion only of said voltage divider such that a bias voltage is applied to certain of said grids of sufficient magnitude to terminate the operation of said tubes.

2. A bias voltage supply and keying control unit for use with a multi-grid oscillator tube, comprising a source of direct current potential at least equal to the maximum bias voltage required for any of said grids, a voltage divider network connected across said source, connections from said source adapted for connection to a pair of such, grids, circuit closing means for selectively short-circuiting a portion only of said voltage divider, and means for connecting. an intermediate point on said network to the cathode of such a tube.

3. A bias voltage supply and keying control unit for use with an oscillator tube of the type having at least an anode, a cathode, a control grid and a screen grid, comprising a source of direct current potential at least equal to the maximum bias voltage required for either of said grids, a voltage divider network connected across said source, connections from said source adapted for connection respectively to said grids, and circuit closing means for selectively short-circuiting a portion only of said voltage divider such that a biasing voltage is applied to each of said grids.

4. A bias voltage supply and keying control unit for use with an oscillator tube of the type having at least an anode, a cathode, a control grid and a screen grid, comprising a source of direct current potential at least equal to the maximum bias voltage required for either of said grids, a voltage divider network connected across said source, bias resistors connected to respective ends of said network, connections from each of said resistors adapted for connection respectively to said grids, and circuit closing means for selectively short-circuiting a portion only of said voltage divider.

5. A bias voltage supply and keying control unit for use with an oscillator tube of the type having at least an anode, a cathode, a control grid and a screen grid, comprising a source of direct current potential at least equal to the maximum bias voltage required for either of said grids, a voltage divider network connected across said source, bias resistors connected to respective ends of said network, connections from each of said resistors adapted for connection respectively to said grids, circuit closing means for selectively short-circuiting a portion only of said voltage divider, and means for connecting an intermediate point on said network to the cathode of such a tube.

6. In combination, a tetrode oscillator tube comprising an anode, a cathode, a control grid and a screen grid, and a combined grid voltage supply and keying unit, said unit comprising a source of direct current potential, a pair of voltage dividing resistors connected in series across said source, means for connecting the common central terminal of said resistors to said cathode, biasing resistors connected respectively between the terminals of said source and said grids, and contact means for selectively shortcircuiting that one of said series-connected resistors whose outer terminal is connected through its biasing resistor to the control grid of said tetrode.

'7. In combination, an oscillator tube comprising an anode, a cathode, a control grid and a screen grid, a source of direct current potential for applying a potential to said anode which is positive with reference to said cathode, a source of bias Voltage normally operative to apply to said screen grid 2, high positive potential swinging downward to less positive during oscillation of said tube and to apply to said control grid a negative bias voltage with reference to said cathode, and means for simultaneously increasing the negative value of said control grid bias voltage and decreasing the magnitude of the positive potential applied to said screen grid, such that the oscillation of said tube is terminated.

8. In combination, an oscillator tube having at least an anode, a cathode, a control grid and a screen grid, a source of direct current potential for applying a potential to said anode which is positive with reference to said cathode, a source of bias voltage normally operative to apply to said screen grid a, high positive potential swinging to downward to less positive during oscillation of said tube and to apply to said control grid a negative bias voltage with reference to said cathode, a voltage dividing network connected across said source of bias voltage, and means including a pair of contacts connected across a portion of said network and operative upon closure thereof to simultaneously increase the negative value of said control grid bias voltage and to decrease the magnitude of the positive potential applied to said screen grid, to the extent necessary to terminate oscillation of said tube.

TAYLOR A. BIRCKHEAD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,563,342 Conrad Dec. 1, 1925 1,855,509 Blair Apr. 26, 1932 2,223,049 Reichle Nov. 26, 1940 2,259,121 Temple Oct. 14, 1941 2,386,844 Davis Oct. 16, 1945 

